Lateral double-diffused MOSFET (abbreviated as LDMOS) is a crucial technology to high voltage integrated circuits and power integrated circuits, wherein MOSFET stands for metal oxide semiconductor field effect transistor. LDMOS is characterized in that between a trench region and a drain region is arranged a relatively long lightly-doped drift region which is doped in the same way as the drain region. Arranging the drift region may increase a breakdown voltage thereof.
A super-junction LDMOS (abbreviated as SJ LDMOS) is an improved LDMOS, in which a group of n-type pillars and p-type pillars arranged in an alternate manner replace a lightly-doped n-type drift region of a traditional LDMOS. Theoretically, with the charge compensation between p-type pillar and n-type pillar, SJ LDMOS may have a high breakdown voltage, and meanwhile the heavily-doped n-type pillars may have a very low on-resistance. Therefore, super-junction devices may achieve a good balance between the breakdown voltage and the on-resistance. However, the substrate-assisted depletion effects result in a decrease in the breakdown voltage of the SJ LDMOS device.
The substrate-assisted depletion effects refer to that lateral super junctions are affected by a vertical electric field such that the symmetrical p-type pillars and n-type pillars in the super junctions may not be completely depleted at the same time, that is to say, the charge balance between the p-type pillar and the n-type pillar is destroyed. For a silicon-on-insulator (SOI) substrate in off-state, the back-gate effect of the substrate motivates non-uniformly distributed charges to accumulate at the interface between the buried oxide layer and the bottom silicon film by the action of the vertical field, enlarging the charge difference between the p-type pillar and the n-type pillar. This thereby results in that the p-type pillars and the n-type pillars fail to be completely depleted at the same time at a theoretically-calculated breakdown voltage.
There are generally two solutions to eliminate the substrate-assisted depletion effects of the SJ LDMOS.
One solution is to use a complete insulation substrate such as a sapphire substrate. As another example, a substrate obtained by etching a SOI substrate and filling epoxy resin into the thus-etched vacant cavity may be used. This solution may enable the substrate-assisted depletion effects to be completely eliminated, but its process is complicated and inordinately thin silicon increases on-resistance of devices.
Another solution is to prepare the SJ LDMOS on a common substrate (such as bulk silicon or SOI), and the charge balance between the pillars is achieved by, for example, designing the super junctions to be conical, controlling a width of the column, blending a SJ structure and a reduced surface field (RESURF) structure, or introducing a buffer layer. However, this solution is unfavorable for its failure to control distribution of impurities in the pillars and to realize the charge balance at the entire drift region.